In spermatogenesis, undifferentiated male germ cells undergo meiosis to produce haploid spermatozoa. This process is extremely complex and a disruption at any stage can result in spermatogenic failure-a leading cause of male infertility tha affects roughly 1 in 100 men worldwide. While spermatogenic failure has been intensively researched for several decades, the underlying genetic cause can be determined in only about 30% of cases. For several reasons, I hypothesize that a substantial fraction of these unexplained cases are due to novel mutations that occur in genes (or in regulatory regions) located on the X and Y chromosomes. First, a large fraction of genes on these chromosomes are expressed in the testis and thus are likely to function in male specific processes like spermatogenesis. Second, the X and Y chromosomes are present in only one copy in males and so mutations that occur here can result in dramatic phenotypes like spermatogenic failure because no functional allele exists to compensate for the mutation. In this proposal, I describe a series of experiments to test this hypothesis and identify genes that are important for male fertility. To provide a better understanding of human spermatogenesis and to determine the suite of genes that are its key regulators, I will first profile global gene expression across isolated spermatogenic cell types that correspond to distinct phases of spermatogenesis. I will then identify mutations that are responsible for male infertility by resequencing the sex chromosomes of men with nonobstructive azoospermia, a condition where no sperm is detected in their semen. The results of this research will enhance our understanding of human spermatogenesis and improve our ability to diagnose male infertility in the clinic.